I mention it in this forum because
a) It's a cool picture, and
b) There is some physics in it.
I recommend this topic as an answer to any student who
gets the idea that the frontiers of physics have receded
to the point where ordinary mortals cannot understand the
questions, let alone the answers.
This is an example where the question can be understood
by anyone: How is lightning produced?
The answer is probably not even very complicated. We just
don't know what it is.
Some background:
In 1960, Feynman devoted an entire lecture to "Electricity
in the Atmosphere". The physics of lightning was not well
understood back then ... and it is *still* not well
understood.
The "conventional" pseudo-theories rely on the properties of
H2O, and in particular the splitting of polarized droplets, http://en.wikipedia.org/wiki/Lightning
Such things are fun to build. Be a little big careful,
because they work too well. They can rapidly build up a
humongous charge, if you arrange the details just right.
However, any H2O-based theory of lightning is dead on arrival
if we are trying to explain the lightning associated with a
volcanic ash plume.
My skepticism of the drop-splitting theories is compounded
by the fact that I've seen plenty of lightning inside _snow_
storms where there is no reason to believe any droplets are
being split.
One thing that all lightning-producing scenarios have in
common is violent turbulence. We know that lightning is
more common in rainstorms than snowstorms, but one could
easily conjecture that this is because the latent heat
of condensation (vapor -> liquid) is very large. In a
snowstorm, the latent heat of aggregation (vapor -> solid)
is larger on a _per mole of water_ basis, but only slightly
so, and the overall effect is smaller _per mole of air_
because the temperature is lower and therefore the absolute
humidity is much much lower. The latent heat is relevant
because it is the energy source that drives the turbulence.
A volcano produces plenty of turbulence in the absence of
precipitation, and indeed in the absence of any solid or
liquid H2O.